This invention relates to filament power supplies for display tubes, and more particularly to such supplies for powering filaments in the context of multiple scan frequencies.
Video display tubes such as television picture tube are vacuum tubes which ordinarily use a filament to heat the cathode in order to free electrons from the cathode. The electrons from the cathode are accelerated toward the ultor or phosphor faceplate to generate a luminescent dot. The dot is scanned over the face of the picture tube, which creates a bright display. The intensity of the electron beam is modulated in consonance with the scanning to create a picture.
In the past, television or video picture tubes have been supplied with voltage from a horizontal deflection transformer driven from a regulated scan voltage at the horizontal scan frequency, which pursuant to National Television Standards Committee (NTSC) standards corresponds to about 15,734 Hz. The high voltage for the picture tube ultor is commonly generated by rectifying the voltage from a high voltage secondary winding of a deflection transformer, and the filament of the picture tube was driven by alternating voltage from a filament secondary winding of the transformer. The regulation of the scan voltage was believed to be sufficient to maintain the filament voltage within the nominal filament supply voltage range of the picture tube. If the filament voltage deviates from the nominal value, picture tube life can be adversely affected.
The advent of multi-function high definition television (HDTV) and NTSC display systems requires that the display system be operable at both the NTSC standard deflection frequency and another, higher, HDTV deflection frequency. When a picture tube is to be used with both NTSC and with HDTV, the deflection frequency used for NTSC is often 2H, where H represents the NTSC deflection frequency. The HDTV deflection frequency used for this purpose may be, for example, 2.14H. Because of the differing horizontal scan frequencies of HDTV and NTSC systems, a picture tube scanned at those disparate frequencies would, in the absence of compensation for the effects of the deflection frequencies, scan different portions of the phosphor screen, depending upon the scan frequency. In order to cause the scanned portion of the phosphor screen to be the same when displaying NTSC or HDTV, the scan voltage may be increased in proportion to the scan frequency. It should be noted that, by contrast with HDTV, video pursuant to NTSC standards is often referred to as having xe2x80x9cstandard definition.xe2x80x9d
When two different scan voltages are applied to the deflection transformer, depending upon whether the video is standard definition or HDTV, the voltage produced by the filament winding of the deflection transformer varies.
One possible way to regulate the filament voltage of a television display tube is to produce alternating filament voltage, rectify the alternating voltage to produce pulsating voltage or current, and to filter the pulsating current by the use of capacitors, to thereby produce direct voltage. In this context, the term xe2x80x9cpulsatingxe2x80x9d differs from xe2x80x9calternatingxe2x80x9d in that pulsating voltage or current is principally unidirectional, while alternating generally suggests voltages taking opposite polarities relative to zero and currents flowing in generally equal amounts in both directions. The direct voltage produced by rectification and filtration can then be regulated in a conventional manner to produce the desired value of filament voltage. This approach is effective, but the cost may be greater than desired, because filament currents in video display devices may be on the order of xc2xe ampere, and the capacitors required for filtering may be large, expensive, and subject to aging. The active devices required for regulating voltage at the required currents tend to be expensive because of the relatively high power being controlled, and their mountings also tend to be expensive because of the heat to be removed.
According to an aspect of the invention, a controllable coupling device is coupled between the filament voltage winding of a deflection transformer and the filament(s) of a display tube, to control the magnitude of the coupling under the control of a control signal, and the control signal is selected to provide the same filament voltage regardless of the applied scan voltage. The controllable coupling device may be viewed as a variable voltage divider, a variable load, a variable current shunt, or a level shifter.
A video display power supply for energizing a filament load of a cathode ray tube according to an aspect of the invention comprises a source of an input supply voltage, and also comprises a switching semiconductor coupled to the input supply voltage source for generating an alternate current, unregulated supply output at a frequency related to a deflection frequency. The unregulated supply output is applied to the filament load unfiltered, with respect to the frequency of the unregulated supply output, to develop in the filament load an unfiltered, second supply output. The video display power supply also includes a regulator for regulating the second supply output, and a source of regulation control signal. The regulation control signal has a value selected from a range of values, and is coupled to an input of the regulator for regulating the second supply output in accordance with the selected value of the control signal.
According to another aspect of the invention, a video display power supply energizes a filament load of a cathode ray tube. The video display power supply according to this other aspect of the invention comprises a power supply power stage for generating an unregulated supply output voltage at a frequency related to a deflection frequency, where the deflection frequency being selected from a range of frequencies. A regulator applies the unregulated supply output voltage to the filament load to generate in the filament load a regulated supply output voltage. A source of regulation control signal is coupled to a control input of the regulator and has a selectable first value when a first deflection frequency is selected and a selectable second value when a second deflection frequency is selected for regulating the unregulated supply output voltage in accordance with the selected deflection frequency.